Pollen will last forever if stored properly. The important bit is to dry the pollen before storing. Fresh pollen will only last a day or two before it is useless. It can last quite a bit longer if refrigerated.

Collect the pollen in a glass jar or baggie. Add a silica drying pouch, you know, those keep fresh pouches that come with anything that needs to remain dry, like vitamins. Close the lid. It will dry out in a few hours depending on the quantity. Once dry, put it in a storage container (baggie will do) and put it in the freezer. It will keep for years and years.